Understanding how species form is one of the fundamental unanswered questions of evolutionary genetics. And it is critical in understanding genome evolution. Answering this question requires knowledge of the genetic basis of reproductive isolation between species. Specific questions are: (1) what evolutionary processes are responsible for reproductive barriers between species?; (2) what are the frequency of certain barriers to reproduction between species'?.; (3) how many loci contribute to the quantitative traits associated with reproductive isolation between species?; and (4) what genes control reproductive isolation between species and how do they function? To begin to answer these questions, the ecological and genetic basis of reproductive isolation between two species in the plant genus Leavenworthia (L. alabamica and L. crassa) will be determined. Reciprocal transplant experiments will determine whether or not ecogeographic isolation is present. Pollinator observations will be conducted to see if pollinator preferences differ between species, which would be a form of ethological isolation. Leavenworthia alabamica and L. crassa hybridize in nature, and thus hybrid viability will be examined in order to determine if any post-zygotic barriers to reproduction exist. Quantitative trait loci will be mapped to determine how many genetic loci are responsible for reproductive isolation. These QTL will be compared to the sequence of Arabidopsis thaliana as a clue to gone function. Finally, the genetics of any inbreeding depression observed in these species or their hybrids will be determined.